1. Field of the Invention
The present invention relates to a noise shaper used in a xcex94-xcexa3 (delta-sigma) digital-to-analog (D/A) converter, digital-to-digital (D/D) converter, or the like and, more particularly, to a noise shaper for suppressing the idle pattern by means of a dither signal.
2. Description of the Related Art
A noise shaping technique consisting of reconverting an oversampled signal into a signal of a less number of bits by a quantizer and feeding quantization noise induced by the requantization back into the next signal to shift the quantization error to the higher-frequency side for reducing noise within the signal band is used in xcex94-xcexa3 (delta-sigma) D/A conversion and D/D conversion. Where this noise shaping is implemented, an idle pattern having a certain pattern at quite low levels of the input signal is produced. An intrinsic spectrum is produced within the audio band, thus deteriorating the sound quality.
In the past, therefore, a dither generator for supplying a quite low level of dither signal to an input signal at high frequencies has been used. For example, in Japanese Patent Laid-Open No. 102851/1993, a D/A converter for adding a dither signal to a signal that is input to a noise shaper 81 as shown in FIG. 8 is disclosed, the dither signal being generated by a dither generator circuit 82.
In the field of digital audio technology, noise shapers of higher orders have been used because further improvement of sound quality has been required. Concomitantly, noise components at higher frequencies have increased. This has placed a great load on the rear-stage, low-pass filter. To solve this problem, an attempt has been made to suppress noise components at high frequencies by the use of a pole-shifted noise shaper in which the pole on the Z-plane of the transfer function of the whole system has been shifted out of the origin. As illustrated in FIG. 9, this system implements feedback to an adder 92 from a quantizer 91 by a transfer function G(Z). This is subtracted from the input signal. Thus, feedforward is provided to the quantizer 91 with the transfer function G(Z). The output data Y(n) is herein abbreviated Y and given by
Y=(Xxe2x88x92Yxc2x7H)xc2x7G+Q
where H is a transfer function H(Z), G is a transfer function G(Z), X is input data X(n), and Q is quantization noise in a quantizer X1. Thus, we have   Y  =                    G                  1          +                      G            ·            H                              ⁢      X        +          Q              1        +                  G          ·          H                    
It can be seen from this that a pole shift has occurred and that the input signal attenuates with increasing the frequency. One example of a pole-shifted noise shaper is shown in FIG. 10. This is a third-order noise shaper consisting of integrators 101-103, adders 104-107, coefficient multipliers 108-111, and a quantizer 112. Feedback is provided to the input side from the quantizer 112. In addition, feedback is provided to the integrator 102 from the integrator 103 via the adder 107 and via the coefficient multiplier 111.
In the pole-shifted noise shaper, the input signal attenuates with increasing the frequency. Therefore, if the signal to which the dither signal is added as shown in FIG. 8 is subjected to noise shaping, the dither signal attenuates. Consequently, the idle pattern cannot be suppressed sufficiently.
Furthermore, noise components at higher frequencies are increased by adding the dither signal. This increases the load on the low-pass filter positioned behind the noise shaper. For this reason, if attempts are made to reduce noise components at higher frequencies using a pole-shifted noise shaper, the effect is canceled to some extent.
The present invention provides a pole-shifted noise shaper in which the pole on the Z-plane of the transfer function has been shifted out of the origin, the noise shaper being characterized in that it is equipped with a dither adder for adding a dither signal to data that is about to be applied to a quantizer. In consequence, the idle pattern can be suppressed more effectively.
Preferably, the above-described dither signal varies periodically regardless of the aforementioned data, and its spectrum is based on a certain frequency that is higher than the audio band and lower than the operating clock frequency.
Preferably, the dither signal is produced based on a certain pattern consisting of combinations of xe2x80x9c1xe2x80x9d and xe2x80x9c0xe2x80x9d. The dither adder described above sequentially selects the bits of the certain pattern at the certain frequency, adds a certain level to them, and produces the dither signal.
It is desired that the certain frequency described above be {fraction (1/16)} to xe2x85x9 of the operating clock frequency of the noise shaper.
The level of the above-described dither signal is preferably {fraction (1/16)} to xe2x85x9 of the maximum amplitude, or the most significant bit, of the data described above.
Preferably, the aforementioned noise shaper converts a multi-bit digital signal into a 1-bit digital signal.